Oxygen Redox Process Research Articles

Relationship between rate performance and electronic/structural changes during oxygen redox of lithium-rich 4d/3d transition metal oxides

Li-rich solid solution cathode materials having 3d transition metals or 4d transition metals are the candidate of next generation cathode material. In order to clarify the rate performance of 3d or 4d transition metal oxides, we focused two typical Li-rich solid solution cathodes; Li1.2Ni0.13Co0.13Mn0.54O2 as a model of 3d transition metal oxide and Li1.2Ni0.13Co0.13Ru0.54O2 cathode as a model solid solution cathode containing 4d transition metal. We have applied operando hard and soft X-ray absorption spectroscopies clarify electronic and local distortion of both cathodes. From the results of XAS measurements, the reaction mechanism of Li1.2Ni0.13Co0.13Ru0.54O2 cathode was clearly explained by reversible transition metal redox reaction without the oxygen redox process. The EXAFS analysis suggests that Li1.2Ni0.13Co0.13Ru0.54O2 cathode also had the small distorted structure around Ru compared with around Mn of Li1.2Ni0.13Co0.13Mn0.54O2 cathode. The small distorted structure takes advantages for Li+ diffusion, resulting in the high rate performance of the Li1.2Ni0.13Co0.13Ru0.54O2 cathode.

Fluorination effect for stabilizing cationic and anionic redox activities in cation-disordered cathode materials

Cation-disordered Li-excess cathodes with oxygen redox reactions are promising candidates for high-energy-density Li ion batteries. Nevertheless, the oxygen redox process that is required for the high capacity often comes with the oxygen loss, which leads to severe capacity degradation and voltage decay. In this work, we have successfully synthesized a series of Li-excess cation-disordered cathodes (Li1.2Mn0.4+xTi0.4-xO2-xFx) (0 ​≤ ​x ​≤ ​0.2) with different fluorine (F) contents. The electrochemical performance results show that the Li1.2Mn0.55Ti0.25O1.85F0.15 (LMTOF0.15) exhibits the highest reversible capacity (275 mAh g-1, under 30 ​mA ​g-1), cyclability, and voltage retentions. The mapping of resonant inelastic X-ray scattering (mRIXS) and differential electrochemical mass spectroscopy (DEMS) results reveal that the fluorination enhances the reversible lattice oxygen redox reaction while suppressing irreversible gas release and surface reactions. The X-ray Absorption Spectroscopy (XAS) during the initial two cycles shows that F-substitution alleviates the reduction of the Mn valence state during the whole (dis)charge processes in the bulk and at the surface of the material, results in higher average discharge voltage. In addition, the introduction of F improves the structural stability and suppresses local lattice distortion of the material. Therefore, LMTOF0.15 is able to cycle with smaller polarization, less interfacial side reaction and Mn dissolution, and therefore results in enhanced cyclability. This work provides a comprehensive understanding of the fluorination effect on the cationic and anionic redox activities in cation-disordered Li-excess cathodes.

Heteroepitaxial oxygen-buffering interface enables a highly stable cobalt-free Li-rich layered oxide cathode

Abstract The oxygen redox process plays an essential role for the high charge-discharge capacity in Li-rich layered oxide (LLO) cathodes. The irreversible release of lattice oxygen may lead to surface reconstruction and cathode-electrolyte interfacial reactions, transition metal (TM) dissolution, as well as microcrack evolution, etc. during cycling that limit the commercial application of LLO cathodes. Herein, we propose the design of a heteroepitaxial Fluorite(CeO2)@Rocksalt@Layered interface with oxygen buffering effects in Cobalt-free Li1.2Mn0.53Ni0.27O2 through the incorporation of ceria. Experimental characterization and theoretical calculations reveal that the fluorite CeO2 nanolayer with oxygen vacancies suppresses the irreversible lattice oxygen loss and cathode-electrolyte interfacial reactions in LLOs. Moreover, the synergy involving the formed rocksalt interphase and Ce3+ doping in the bulk not only stabilizes the structural integrity, resulting in substantial enhancement of capacity/voltage retention, but also accelerates the electrochemical kinetics upon cycling. This finding may pave the path for utilizing the reversible oxygen redox process and designing new high capacity TM-oxide cathode materials.

Reversible Anionic Redox Activities in Conventional LiNi1/3Co1/3Mn1/3O2 Cathodes

AbstractRedox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium‐excessive layered‐oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi1/3Co1/3Mn1/3O2. The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high‐capacity lithium‐ion batteries.

High Capacity Prismatic Type Layered Electrode with Anionic Redox Activity as an Efficient Cathode Material and PVdF/SiO2 Composite Membrane for a Sodium Ion Battery.

A prismatic type layered Na2/3Ni1/3Mn2/3O2 cathode material for a sodium ion battery is prepared via two different methods viz., the solid state and sol–gel method with dissimilar surface morphology and a single phase crystal structure. It shows tremendous electrochemical chattels when studied as a cathode for a sodium-ion battery of an initial specific discharge capacity of 244 mAh g−1 with decent columbic efficiency of 98% up to 250 cycles, between the voltage range from 1.8 to 4.5 V (Na+/Na) at 0.1 C under room temperature. It is much higher than its theoretical value of 173 mAh g−1 and also than in the earlier reports (228 m Ah g−1). The full cell containing this material exhibits 800 mAh g−1 at 0.1 C and withstands until 1000 cycles with the discharge capacity of 164 mAh g−1. The surpassing capacity was expected by the anionic (oxygen) redox process, which elucidates the higher capacity based on the charge compensation phenomenon.

How Bulk Sensitive is Hard X-ray Photoelectron Spectroscopy: Accounting for the Cathode-Electrolyte Interface when Addressing Oxygen Redox.

Sensitivity to the "bulk" oxygen core orbital makes hard X-ray photoelectron spectroscopy (HAXPES) an appealing technique for studying oxygen redox candidates. Various studies have reported an additional O 1s peak (530-531 eV) at high voltages, which has been considered a direct signature of the bulk oxygen redox process. Here, we find the emergence of a 530.4 eV O 1s HAXPES peak for three model cathodes-Li2MnO3, Li-rich NMC, and NMC 442-that shows no clear link to oxygen redox. Instead, the 530.4 eV peak for these three systems is attributed to transition metal reduction and electrolyte decomposition in the near-surface region. Claims of oxygen redox relying on photoelectron spectroscopy must explicitly account for the surface sensitivity of this technique and the extent of the cathode degradation layer.

Both cationic and anionic redox chemistry in a P2-type sodium layered oxide

The demand for high energy Na-ion batteries has promoted intensive research on high energy oxygen redox chemistry in layered transition metal oxide cathodes. However, most layered cathodes with oxygen redox might suffer from irreversible electrochemical reaction, fast capacity decay and underlying O2 release. Herein, we report that copper element with a strong electronegativaty can stablize Na-deficient P2-Na2/3Mn0.72Cu0.22Mg0.06O2 phase to achieve both cationic and anionic redox chemistry. Hard and soft X-ray absorption spectra demonstrate that all Mn3+/Mn4+, Cu2+/Cu3+ and O2−/(O2)n− participate in the redox reaction upon Na+ ions extraction and insertion. Density functional theory (DFT) calculations confirm that the strong covalency between copper and oxygen ensures the cationic and anionic redox activity in P2-Na2/3Mn0.72Cu0.22Mg0.06O2 phase. The P2-Na2/3Mn0.72Cu0.22Mg0.06O2 cathode could deliver stable cycling life with 87.9% capacity retention at 1C during 100 cycles, as well as high rate performance (70.3 mA h g−1 cycled at 10C). Our findings not only provide a promising guidelines to enhance the electrochemical performance of layered oxides based on anionic redox activity, but also explore the potential science behind oxygen redox process.

Stabilizing the oxygen lattice and reversible oxygen redox in Na-deficient cathode oxides

To overcome the limitation of capacity from traditional oxide-based intercalation cathodes, oxygen redox reaction garners intense interest in Li-rich/Na-rich cathodes for rechargeable batteries. However, cycle degradation and voltage drop caused by the irreversible structural changes and O2 release upon electrochemical cycling hinder their commercial applications. Herein, a Na-deficient layered compound Na0.67Mn0.72Zn0.28O2 is employed to study the relationship between the oxygen redox reaction and the (O2)n− stabilization mechanism by using a combined experimental and computational approach. Our results indicate that Na0.67Mn0.72Zn0.28O2 has a long plateau of approximately 4.1 V with a total capacity of 157 mAh g−1, majorly stemming from the oxygen redox reactions of O2−/(O2)n−. The O-2p nonbonding states originating from the weak Zn–O interaction and the shortened O–O bonds induced by the absence of Na atoms in the alkali layers catalyze the formation of peroxo-like O–O dimer between the Mn–Mn atoms. The honeycomb TM layer of the Na-deficient materials remains stable with a small O–O bond length evolution from 2.55 Å to 2.48 Å during the oxygen redox process, which stabilizes the lattice oxygen in electrochemical cycles. This finding is important for further understanding the mechanism underlying the oxygen redox chemistry of high energy Na-deficient cathode materials.

Probing the thermal effects of voltage hysteresis in anionic redox-based lithium-rich cathodes using isothermal calorimetry

Probing the thermal effects of voltage hysteresis in anionic redox-based lithium-rich cathodes using isothermal calorimetry

Reversible Oxygen Redox Chemistry in Aqueous Zinc-Ion Batteries.

Rechargeable aqueous zinc-ion batteries (ZIBs) are promising energy-storage devices owing to their low cost and high safety. However, their energy-storage mechanisms are complex and not well established. Recent energy-storage mechanisms of ZIBs usually depend on cationic redox processes. Anionic redox processes have not been observed owing to the limitations of cathodes and electrolytes. Herein, we describe highly reversible aqueous ZIBs based on layered VOPO4 cathodes and a water-in-salt electrolyte. Such batteries display reversible oxygen redox chemistry in a high-voltage region. The oxygen redox process not only provides about 27 % additional capacity, but also increases the average operating voltage to around 1.56 V, thus increasing the energy density by approximately 36 %. Furthermore, the oxygen redox process promotes the reversible crystal-structure evolution of VOPO4 during charge/discharge processes, thus resulting in enhanced rate capability and cycling performance.

Reversible Oxygen Redox Chemistry in Aqueous Zinc‐Ion Batteries

AbstractRechargeable aqueous zinc‐ion batteries (ZIBs) are promising energy‐storage devices owing to their low cost and high safety. However, their energy‐storage mechanisms are complex and not well established. Recent energy‐storage mechanisms of ZIBs usually depend on cationic redox processes. Anionic redox processes have not been observed owing to the limitations of cathodes and electrolytes. Herein, we describe highly reversible aqueous ZIBs based on layered VOPO4 cathodes and a water‐in‐salt electrolyte. Such batteries display reversible oxygen redox chemistry in a high‐voltage region. The oxygen redox process not only provides about 27 % additional capacity, but also increases the average operating voltage to around 1.56 V, thus increasing the energy density by approximately 36 %. Furthermore, the oxygen redox process promotes the reversible crystal‐structure evolution of VOPO4 during charge/discharge processes, thus resulting in enhanced rate capability and cycling performance.

(Plenary) Anion Redox in Metal Oxides

Traditional Li-ion positive electrodes such as layered LiTMO2 (TM = Co, Ni, Mn) store lithium ions and electrons largely by employing the cationic redox of the transition metal (TM), yielding a capacity of ~140 mAh g-1 upon charging to 4.2 VLi. Recent advances on Li-overstoichiometric layered materials Li1+x TM 1−yO2 (TM = Ti, Ni, Co, Mn, Ru, Ir) offer considerably greater first-cycle capacities than the conventional layered materials LiMO2 with over than 200 mAh g-1.1-4 The extra capacity can only be rationalized through the activation of anionic redox in those chemistries, in addition to conventional cationic redox mechanism. However, by tuning transition metal species, researchers observed different reversibility of the anionic redox process, inducing drastically distinct capacity fade over charge and discharge. 1, 3-4 To fully utilize and harvest the extra capacity from oxygen redox process, a clear understanding the redox behavior of those materials is needed. In the past decades, there are several competing hypotheses proposed on the redox process of Li-rich layered materials5-7, but a governing anionic redox mechanism is still lacking. In this work, we employed X-ray core-level spectroscopies, coupled with ab-initio calculations to first understand the redox processes and structural evolution of model systems exhibiting oxygen redox. Through this effort, we have presented a holistic picture of the anionic redox mechanism in the context of previously proposed redox schemes put forward by different researchers. Moreover, through leveraging the learning of the redox process of model materials, we systematically tuned the transition metal species in the metal oxygen framework in those Li-rich oxides, which allows us to pinpoint potential electronic-structure-based descriptors to capture the activation and reversibility of the anionic redox process. This study has laid the foundation for future high-throughput screening of new generation high-energy-density positive electrodes for Li-ion batteries.

Origin of charge compensation and its effect on the stability of oxide cathodes for Li-ion batteries: The case of orthosilicates

The charge-compensation mechanism and host stability of a cathode play the central role in determining its reversible capacity. Here, first-principle calculations are presented to study the charge compensation and its effect on the stability of orthosilicates, Li2−xTMSiO4 (TM = Fe, Mn), as the promising high-capacity cathode materials for Li-ion batteries. The charge compensation in Li2−xTMSiO4 upon delithiation is found to be achieved first by a combined reversible TM and oxygen redox process, originating from the dynamic response of their electronic structures to the Li ions (or electrons) removal and the associated charge transfer from the O to Fe ions, and then by the irreversible formation of O vacancy (VO) that destroys the host stability of these materials. Whether the formation of VO in these materials upon delithiation would occur is demonstrated to be essentially determined by the energy level of their highest occupied electronic states and can be understood by the defect charge transition mechanism which provides a quantitative way to estimate to what extent the oxygen redox could be reversibly used in a cathode that being important for the future design of high-capacity cathode materials.

A High-Capacity O2-Type Li-Rich Cathode Material with a Single-Layer Li2 MnO3 Superstructure.

A high capacity cathode is the key to the realization of high-energy-density lithium-ion batteries. The anionic oxygen redox induced by activation of the Li2 MnO3 domain has previously afforded an O3-type layered Li-rich material used as the cathode for lithium-ion batteries with a notably high capacity of 250-300 mAh g-1 . However, its practical application in lithium-ion batteries has been limited due to electrodes made from this material suffering severe voltage fading and capacity decay during cycling. Here, it is shown that an O2-type Li-rich material with a single-layer Li2 MnO3 superstructure can deliver an extraordinary reversible capacity of 400 mAh g-1 (energy density: ≈1360 Wh kg-1 ). The activation of a single-layer Li2 MnO3 enables stable anionic oxygen redox reactions and leads to a highly reversible charge-discharge cycle. Understanding the high performance will further the development of high-capacity cathode materials that utilize anionic oxygen redox processes.

Rotating Ring Disk Electrode for Monitoring the Oxygen Release at High Potentials in Li-Rich Layered Oxides

Li-rich layered oxides show a staggering capacity that relies on cumulative cationic and anionic redox processes. However, their practical applications are plagued by roadblocks dealing with large hysteresis, capacity fade and irreversible oxygen loss during the first charge that causes undesirable structural changes. Hence, the first step to screen the Li-rich layered oxides is the identification of this gas release phenomenon and its better understanding. Online electrochemical mass spectrometry (OEMS) is presently used for the elucidation of gas evolution, but its usage is lengthy and far to be routine. Herein we propose the utilization of the simple rotating ring disc electrode (RRDE) technique for the identification of O2 release phenomenon and hence the quick screening of Li-rich layered oxides. We have evaluated the feasibility of RRDE to monitor the O2 generation by conducting studies on Li-half cells having various Li-rich layered oxides as cathodes and found that our results nicely compare with those obtained by OEMS, hence demonstrating the validity of RRDE technique. The proposed RRDE approach which offers high sensitivity for the identification of O2 release while being fast and simple should greatly help to advance the understanding of oxygen redox processes and promote the design of new materials.

Oxygen redox processes in PEGDME-based electrolytes for the Na-air battery

We present the characterization by the rotating-disc-electrode (RDE) and cyclic-voltammetry (CV) methods of oxygen-reduction/oxygen-evolution (ORR/OER) processes in the Na-air battery containing fresh and aged polyethylene glycol dimethyl ether 500 (PEGDME500)-based electrolyte. It was found that the time of storage of this solvent affects the oxidation overpotentials of the reduction products. It is suggested that the reason for the PEGDME500 change originates from the synthesis process, in which the sodium alcoholate molecule can remain in the solvent. Insertion of metallic sodium prevents aging of PEGDME500. The results are compared with a tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte. It was shown that the voltammogram of the TEGDME-based electrolyte is similar to that of the fresh PEGDME500, but with the opposite ratio of NaO2-to-Na2O2 oxidation currents. This indicates that Na2O2 is the main reduction product in TEGDME, and we suggest that in fresh PEGDME500, Na2O2 is formed by the chemical disproportionation of the dissolved NaO2 in the solution.

Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials

Recent progress in the understanding of percolation theory points to cation-disordered lithium-excess transition metal oxides as high-capacity lithium-ion cathode materials. Nevertheless, the oxygen redox processes required for these materials to deliver high capacity can trigger oxygen loss, which leads to the formation of resistive surface layers on the cathode particles. We demonstrate here that, somewhat surprisingly, fluorine can be incorporated into the bulk of disordered lithium nickel titanium molybdenum oxides using a standard solid-state method to increase the nickel content, and that this compositional modification is very effective in reducing oxygen loss, improving energy density, average voltage, and rate performance. We argue that the valence reduction on the anion site, offered by fluorine incorporation, opens up significant opportunities for the design of high-capacity cation-disordered cathode materials.

Oxygen Redox in Li-Ion Battery Chemistries

The energy density of lithium batteries is currently restricted by the cathode material, delivering ~ 160-190 mAh g-1.1 New high-energy-density cathode materials are much sought after to meet the increasing requirements of consumer electronics, electric vehicles and grid energy storage. The so called lithium-rich layered oxide materials (e.g. Li1-x[Li0.2Mn0.6Ni0.2]O2), exceed the conventional limit of charge storage as far more lithium (x ≈ 1, corresponding to 310 mAh g-1) can be extracted than can be accounted for through transition metal oxidation (x ≈ 0.4). The source of this “extra” capacity has been explained previously by a number of models including oxygen loss, electrolyte decomposition and anion redox. Recently, several groups have identified that this “extra” capacity is resulting from a reversible oxygen redox process2-7. In this work, we examine in detail the charging process in Li[Li0.2Ni0.2Mn0.6]O2 with direct experimental evidence of a dominant O redox process, accompanied by a minor contribution from oxygen loss, when the material is charged beyond 4.4 V. The formation of hole states around the oxygen during charging was probed using soft x-ray absorption spectroscopy (SAXS), as shown in Figure 1. Additionally, recent results that underpin the oxygen redox process in a number of closely related Li-rich compositions will be discussed, as well as the requirements necessary to form holes on the oxygen in these compounds. The conclusions of this work, and that of other researchers working to understand anion redox, provide us with a guidance which can be used to discover future high energy density cathode materials.

Controlling Li2CuO2 single phase transition to preserve cathode capacity and cyclability in Li-ion batteries

Li2CuO2 is synthesized via a solid-state reaction and its structure and microstructure is characterized using X-Ray Diffraction, Scanning Electron Microscopy and N2 adsorption–desorption isotherms. The capacity of cathode material is evaluated at different preparation conditions to determine the factors affecting charge retention, cyclability, and assuring reproducibility during electrode fabrication. Progressing from previous reports, a maximum capacity retention of 140mAhg−1 is attained in the potential window from 1.5 to 4.2V (Li/Li0) during ten cycles at C/15. The low capacity retention at extended cycling has been associated to the participation of irreversible oxygen redox process evaluated by theoretical calculations and cyclic voltammetry. These processes are minimized when the cycling potential window is confined from 2.0 to 3.8V (Li/Li0), thus, achieving a higher capacity retention up to 100mAhg−1 during 60cycles. Cycling at higher C/rates lowers the capacity down (60mAhg−1 at C/5), but the maximum capacity is restored when returning to C/15. Thus, making Li2CuO2 an attractive material either as active compound or additive in cathodes for Li-ion batteries, as a result of its intrinsic properties such as environmental benign, abundance, cost and straightforward preparation process.

The Origin of the Oxygen Redox Activity in Layered and Cation-Disordered Li-Excess Cathode Materials

With increasing complexity of technology comes a demand for higher-energy density Li-ion batteries, which requires high-energy density cathode materials. Reversible oxygen redox in Li-excess materials can substantially expand the search space of high capacity and energy density cathodes, because it can deliver excess capacity beyond the theoretical transition metal (TM) redox capacity, reducing the necessity of heavy and expensive TM ions.[1-4] Nevertheless, the structural and electronic origin of the oxygen redox process is not understood, preventing the rational design of better cathode materials with oxygen redox. In this talk, we explain how specific chemical and structural features in layered and cation-disordered Li-excess cathode materials introduce labile oxygen electrons that can be easily extracted and participate in the practical capacity of these materials. Our ab initio calculations demonstrate that Li excess and cation disorder create unhybridized oxygen 2p states in lithium-metal-oxide cathodes that promote oxygen redox, which can create extra capacity beyond the TM redox capacity and lead to peroxo-like species when local distortion in the chemical bond is allowed (Figure 1).[5] Furthermore, we explain how this specific oxygen redox process competes with the TM redox, providing clear guidelines for the design of high-capacity cathodes with optimized TM or O redox.